The present invention relates to gas turbine combustors and, more particularly, to a combustor having a liner arrangement capable of withstanding elevated temperatures.
The increasing efficiency of thermal engines with increasing temperature has justified attempts to increase the combustion temperatures in such engines. The principal limitation on the combustion temperature has been the availability of suitable materials to contain the combustion process.
For use in a gas turbine engine, materials have been developed with suitable manufacturable properties capable of withstanding about 1550 degrees F maximum for extended periods of time. At higher temperatures, these materials suffer thermal distress which results in corrosion and/or distorton.
In the prior art, in order to extend the operating temperature of a combustor beyond the capability of the available high-temperature materials, it is known to fabricate gas turbine combustors with relatively complex, and, therefore, undesirable, liner structures supported by a shell.
Furthermore, in advanced systems, the temperature of air available for cooling the combustor is generally increasing. More specifically, and for example, compressor pressure ratios are increasing resulting in higher temperature of the compressor discharge air, for example, about 800 to 1100 degrees F. In advanced systems including a regenerator or recuperator, the compressor outlet air temperature being fed to the combustor through the regenerator may be increased from conventional temperatures to about 1400 to about 1600 degrees F. Thus in these advanced systems, there is an insufficient temperature difference between the compressed air available for cooling and the temperature limit of the materials requiring cooling to maintain the temperature of the combustor liner within a range which can be withstood by conventional combustor materials.
A further trend requiring higher temperature materials in a gas turbine combustor is a move toward higher energy fuels currently not in conventional use in such engines. Some applications, for example, may require the use of a fuel which has a high energy per unit volume. Such fuels may typically consist of a slurry having a liquid hydrocarbon carrier containing carbon and/or powdered metal such as aluminum, boron or zinc. Such fuels contribute increased temperature to the combustor liner in two ways. Typically, high energy slurry fuels have higher flame temperatures than hydrocarbon fuels alone. In addition, such slurry fuels have a much higher radiant emissivity than do conventional hydrocarbon fuels and therefore produce a high radiant flux which transfers thermal energy to the combustor liner. This combination produces a requirement for a combustor liner which can withstand about 2000 to 3000 degrees F.
Although liner materials exist which can withstand higher temperatures, they lack required properties of formability, machinability, weldability and ductility which would permit their fabrication into conventional combustion chamber liners without having relatively complex shapes and attachment arrangements to the remainder of the structure. Several desirable high-temperature liner materials such as certain ceramics and certain fibers in binders can withstand temperatures considerably in excess of 1550 degrees F.
For example, silicon carbide can withstand temperaures as high as about 2800 degrees F.
Another high-temperature material includes a carbon fiber supported in a carbon binder, i.e., carbon-carbon, which can withstand up to about 3000 degrees F. This material must be protected from oxygen by a high-temperature glass or ceramic surface layer to prevent oxidation thereof.
A further high-temperature material includes an oxide dispersion stabilized nickel, chrome alloy, conventionally identified as MA-956 which can withstand temperatures up to about 2100 degrees F.
Conventional combustors typically utilize materials having substantially equal thermal coefficients of expansion for both the liner and the shell structure. This is preferred for reducing thermal stress and strain due to differential thermal expansion and contraction between the liner and its supporting shell.
However, the above-described high temperature liner materials typically have thermal coefficients of expansion which are substantially different than those of conventional shell structures. In a conventional shell-liner arrangement, this would result in increased thermal stress due to differential expansion and contraction. In an arrangement including a ceramic liner, for example, these thermal stresses would cause the brittle ceramic liner to fracture in operation, which is, therefore, not acceptable.